1. Technical Field
The invention relates to the techniques for modulating optical signals, and more particularly to a planar interferometer electro-optic modulator.
2. Discussion
Fiber optic links are becoming increasingly important in a wide variety of applications such as millimeter wave communications and radar systems. An external electro-optic modulator is usually required for a millimeter wave fiber optic link since direct modulation of a solid state laser signal is generally not possible above microwave frequencies.
Travelling wave integrated optic modulators used for this purpose are known in the art. Typically, the modulators include an optical waveguide formed in a substrate and an overlying metallic electrode structure.
The drive frequency applied to the electrode structure is used Examples of such modulators are found in Alferness et al., "Velocity-Matching Techniques for Integrated Optic Travelling Wave Switch/Modulators", IEEE J. Quantum Electronics, Vol. QE-20, No. 3, March 1984, pp. 301-309; Nazarathy et al., "Spread Spectrum Frequency Response of Coded Phase Reversal Travelling Wave Modulators", J. Lightwave Technology, Vol. LT-5, No. 10, October 1987, pp. 1433-1443; and Schmidt, "Integrated Optics Switches and Modulators", from Inteqrated Optics: Physics and Applications, S. Martellucci and A. N. Chester (eds.), pp. 181-210, Plenum Press, New York, 1981.
Travelling wave integrated optic modulators fabricated in substrate materials for which the optical and microwave drive velocities are equal offer the potential of very broad modulation bandwidth. However, for important electro-optic substrate materials such as lithium niobate there is an inherent mismatch between the optical and microwave velocities. Since the optical signal phase velocity in lithium niobate is nearly twice the microwave drive signal velocity, the magnitude of the phase modulation begins to degrade as the phase difference between the optical and microwave drive signals increases. This phenomena is often referred to as phase "walk off".
This velocity mismatch necessitates design trade-offs. The maximum achievable drive frequency decreases as the modulator length is increased. Conversely, to lower the drive voltage and power, a long device length is required. Thus, a trade off must be made between maximum drive frequency and required drive power. In other words, the modulator must be made shorter and the drive power larger as the frequency increases.
In prior attempts to compensate for this velocity mismatch, periodic electrode structures have been used in coplanar electro-optic modulators. These periodic electrode structures can be categorized into either periodic phase reverse electrodes or intermittent interaction electrodes. The known intermittent interaction electrode configurations are unbalanced transmission lines, i.e., they are asymmetric about the propagation axis. This leads to incompatibilities with the balanced line (typically coaxial or waveguide probe) transitions to the other fiber optic link transmitter components. The prior art modulators have been fabricated from Z-cut lithium niobate where the optical waveguide is placed under the metal electrode. In this construction, a dielectric buffer layer is usually required between the metallic electrode and the waveguide in the substrate. The dielectric layer is disadvantageous in that it introduces possible bias point instability. Further, the prior art modulators fail to disclose impedance matching circuitry which would ensure good efficiency and performance of the modulator.